1. Field of the Invention
This invention relates to a method and a system for biological treatment of wastewater.
More specifically, the method and system of the present invention are designed for the biological removal of contamination in the form of solids and soluble organic material, and optionally the biological removal of nitrogen and phosphorus nutrients from wastewater.
2. Discussion of the Prior Art
The biological treatment of sewage and other apparatuses for effecting such treatment are described in U.S. Pat. Nos. 2,907,463, issued to D. J. N. Light et al on Oct. 6, 1959; 4,279,753, issued to N. E. Nielson et al on Jul. 21, 1981; 4,430,224, issued to U. Fuchs on Feb. 7, 1984; 4,431,543, issued to Y. Matsuo et al on Feb. 14, 1984; 4,522,722 issued to E. M. Nicholas on Jun. 11, 1985; 4,663,044, issued to M. C. Goronszy on May 5, 1987; 4,798,673, issued to C. Huntington on Jan. 17, 1989 and 4,948,510 issued to M. D. Todd et al on Aug. 14, 1990, and in Canadian Patents Nos. 997,488, issued to B. K. Tholander et al on Sep. 21, 1976 and 1,117,042, issued to M. L. Spector on Jan. 26, 1982.
In general, suspended growth activated sludge processes and modifications thereto have recently been shown to be able to accomplish the objectives set out above, but the costs of specialized clarifiers required to settle and recycle biological solids is quite high. Moreover, the size, complexity and operating problems encountered with such systems make them unattractive to potential users. In addition, the basic activated sludge process often suffers from poor treatment because of the continuous flow through characteristics in the final clarifier and the resulting constant sludge management requirements.
As a consequence of the foregoing, during the past ten years improved final clarification and solids removal has been sought by using larger and improved but more complex and expensive designs for final clarifiers. Moreover, final effluent filtration is often necessary to reliably meet permitted environmental standards.
There has also been a recent revival of interest in the sequencing batch reactor (SBR) suspended growth activated sludge process because of the inherently more efficient batch settling and higher treatment efficiency possible for batch organic contaminants removal with the SBR process compared to the conventional continuous flow activated sludge process. The SBR process uses the same vessel for batch aerated biological reaction and quiescent batch settling. Thus, the SBR process eliminates the major cost of dedicated final clarifiers necessary for the conventional activated sludge process as well as improving upon solids removal performance.
However, the SBR process has several disadvantages, the principle one being that the process does not operate with continuous flow, but requires intermittent operation for cycles of fill, react, settle, decant and idle. The result can be a much lower volume to use ratio than that achieved using the conventional activated sludge process.
Accordingly, the SBR process is generally not cost effective for flows greater than five to ten MGD. Other disadvantages of the SBR process are that significant head loss occurs from the influent to the final effluent, requiring additional energy and pumping costs, and effluent flow is intermittent which can result in the necessity of constructing flow equalization systems to prevent peak loadings and adverse impacts on receiving waters.
Finally, the basic process and design limitations of the SBR process make it difficult to achieve the same high efficiency biological nutrient removal possible using the continuous flow activated sludge process.
Attempts have been made to overcome the limitations of the SBR and conventional activated sludge processes, while retaining the benefits of both basic processes. As yet, however, none of these attempts have resulted in new processes which have proven to be useful or commercially cost-effective.
The Degremont S.A. U.S. Pat. No. 3,470,092 issued to J. J. P. Bernard on Sep. 30, 1969 illustrates a first attempt to develop a new suspended growth activated sludge process utilizing the concepts of both batch treatment and continuous flow. This two cell process was partially interconnected at the water surface. The alternate cell feed concept was not effective because it did not achieve a high treatment efficiency, had a low aerator utilization factor, and required long detention times to operate resulting in expensive systems. A subsequent Union Carbide U.S. Pat. No. 4,179,366 issued to J. R. Kaelin on Dec. 18, 1979 added a third bottom interconnected cell, but also suffered from low treatment efficiency and ineffective changeover of feeding untreated wastewater from the first cell to the third cell. Moreover, the cross connections between the cells were not effective in achieving a quick-change to a new feeding cycle or in preventing mixing of solids between the cells in a quiescent settling phase and the middle cell undergoing aeration. The processes disclosed by both patents also required significant level fluctuations in the treatment cells between operating cycles which made it difficult to control flows and operate fixed, level sensitive mechanical aeration systems. A later Linde AG German Patent No. 3,147,920 issued on Oct. 13, 1983 utilized the same three cell concept as U.S. Pat. No. 4,179,366, but in order to achieve constant level operations and quicker cycle changes, added large, mechanically operated, full width flap gates between cells. The gates were designed to shut for a period at the end of each outside cell feed cycle in order to hydraulically isolate the cell and permit rapid quiescent settling of suspended solids similar to the SBR batch settling process. The process operated such that during the settling process in the first cell, the untreated wastewater was fed to the middle cell until the opposite end cell was fed.
Although this three cell process achieved a more constant level, and overcame some of the limitations of the prior art, however, the process failed to be useful because it relied on expensive and unreliable mechanical gates to separate the treatment cells at various cycle times. Moreover, the process failed because treatment efficiency and effectiveness was too low to be commercially useful.
A subsequent VOR SA French Patent No. 2550522 issued on Feb. 15, 1985, describes an apparatus including three separate, identical basins. The process concept required the use of a large, low rate suspended growth activated sludge treatment system. Each cell had piping interconnections between each basin so they could operate sequentially in aeration, idle, and clarification modes. This process also required a large, expensive treatment system because three independent basins were required, only 1/3 of the total treatment volume was used for biological treatment at any time, and only 1/3 of the aeration equipment could be used at one time. As a result, this process also has not proven to be useful or cost effective compared to conventional suspended growth activated sludge processes.
In spite of the above described attempts to improve on the performance and effectiveness of conventional activated sludge and SBR activated sludge processes, all such attempts have either failed to achieve the desired benefits, or have new inherent disadvantages which result in little or no net benefits compared to conventional methods. As yet no new processes have been developed which provide a higher treatment efficiency and are commercially more cost effective than the SBR and conventional activated sludge processes.